This invention relates to the art of reducing sulfur dioxide in aqueous medium to form a metal hydrosulfite salt solution. In particular, the invention relates to the manufacture of aluminum hyrosulfite.
Hydrosulfites are commonly produced as aqueous solutions by a metal reduction of sulfur dioxide and/or salts thereof such as sulfite and bisulfites. Metals commonly used commercially are sodium (as mercury amalgams) and zinc (in the form of fine powders). Other metals, including iron and magnesium have also been suggested. Processes for converting zinc hydrosulfite liquors to sodium hydrosulfite are known.
Attempts have also been made to produce aluminum hydrosulfite bleach liquors. It has been demonstrated that aluminum hydrosulfite can be produced from aluminum powder and used as a bleach for paper pulp. Reference is made to a publication of G. W. Smith, et al, Can Pulp Mag., Proceedings Technical Section, D-79, 1971. The results reported in this publication strongly suggest that the aluminum route for manufacture of hydrosulfite would not be amenable for commercial exploitation. For example, in order to obtain the hydrosulfite it was necessary to use aluminum that had been alloyed with environmentally polluting mercury or to use magnesium, which is an expensive metal as the alloy constituent. When an alcoholic medium was employed for the reaction with the mercury-aluminum alloy, the yields based on metal content of the solution were low, and the over-all yield including unreacted metal, were very low, about 10 to 25%. In tests carried out with an aluminum alloyed with about 10% of magnesium, an aqueous reaction medium was used. In these tests, the alloy was first activated with dilute hydrochloric acid (0.6 M) and the dilute acid was used in substantial volume relative to the aluminum powder. Sulfur dioxide gas was then bubbled into the mixture of aluminum and hydrochloric acid and reaction was allowed to proceed for thirty minutes. The yield based on the aluminum consumed was about 70% and the authors postulated that the magnesium cations present in the system may have tended to stablize the hydrosulfite. Over-all yield was only about 25%. The consumption of sulfur dioxide, critical to the operation of a commercially feasible process, was not monitored in any of these experiments. For example, a process which would produce high yields based on the aluminum used or reacted would not be practical for a commercial operation if large excesses of sulfur dioxide would be necessary to secure the high yields.
British Pat. No. 1,007,135 to Goskar discloses a process for bleaching a slurry of iron-contaminated clay by a process in which the clay slurry is treated with an acidified bisulfite and passed through a bed of granulated zinc or magnesium, producing a hydrosulfite that is the bleach reagent. The British patent suggests that a second metal such as aluminum or copper can also be present, the second metal forming an electrocouple with the zinc or magnesium in order to improve the reduction of ferric iron. There was no attempt to produce a bleach liquor in which aluminum hydrosulfite was the sole hydrosulfite species.
To the best of my knowledge, aluminum hydrosulfite bleach liquors are not manufactured on a commercial basis in spite of the fact that it is known that aluminum on a weight basis is more effective as a reducing agent for sulfur dioxide than the metals that are now employed.
Accordingly, a general object of the invention is the provision of a simple, relatively inexpensive process for producing aluminum hydrosulfite liquors useful as a bleach for materials such as clay, which process represents a significant advance over prior art methods for producing aluminum hydrosulfite liquors. In particular, an object of the invention is to provide a method wherein the aluminum that is used does not have to be alloyed with an expensive metal such as magnesium or a polluting metal such as mercury and the yields are high based on both the metal that is reacted and the sulfur dioxide that is used.